Homogenisation is the process of obtaining a uniform composition and stable structure throughout a fluid comprising more than one phase by subjecting the fluid to a fluid mechanical process that involves the subdivision of particles or droplets into small sizes so as to create a dispersion or emulsion.
Homogenising devices, including homogenisation valves suitable for homogenising emulsions, are well know in industry. In essence a homogenisation valve comprises a valve body in which a stationary seat and a spring-loaded plug is located. The plug is biased towards a closed position in which the plug abuts the seat. The plug and/or the seat typically include at least one annular protrusion that abuts the corresponding seat and/or plug, thus forming a narrow homogenisation gap between the plug and the seat.
During homogenisation, a fluid to be homogenised is pumped into an inlet of the valve body under a pressure being sufficient to displace the plug slightly from the seat The fluid subsequently flows through the homogenisation gap, and is subjected to substantial shear forces. As expected, the flow through the homogenisation gap generally exhibits the properties associated with fluid flow between two substantially parallel surfaces, and a shear stress profile develops that ranges from a maximum at the surfaces, and a minimum towards the centre of the flow path. The shear stress induced in the fluid causes bodies such as droplets of the fluid to be homogenised and to be dispersed into smaller droplets. The mass-median droplet diameter of the fluid is thus reduced, which is for instance advantageous when working with emulsions as emulsions are usually of better quality and stability if it has a small mass-median droplet diameter.
In an improved embodiment, a homogenisation valve may include a plurality of annular ridge formations, wherein adjacent ridge formations extend from the plug and seat respectively so as to form a labyrinth or undulating flow path through the homogenisation valve. Pressure recovery regions are therefore formed between adjacent homogenisation gaps, and fluid diverges into these pressure recovery regions once the fluid exits the narrow homogenisation gaps. It will be appreciated that the fluid only diverges in planes substantially normal to the flow through the homogenisation gaps.
One particular homogenisation application is the homogenisation of emulsion explosives. Emulsion explosives have unusual physical properties in that they are made from a large volume of a mobile aqueous phase dispersed in a small volume of a mobile continuous phase usually comprising oils and emulsifiers. These types of compositions tend to have high viscosity, even if the dispersed aqueous phase droplets are not of a very small size. The Reynolds number of the fluid when in motion, being an inverse function of the viscosity, is therefore relatively low, resulting in the flow remaining laminar as there is no transition to a turbulent flow regime. When the fluid is introduced into the homogenisation valve, the flow remains laminar throughout, even when it is accelerated though the homogenisation gap. As described above, the shear stresses peak at the surfaces of the homogenisation gaps, and are lower towards the centre of the fluid flow path. Consequently, large droplets towards edges of the fluid flow path are dispersed into smaller droplets, whereas droplets towards the centre of the fluid remain substantially unchanged.
As described above, the flow remains laminar throughout, and the absence of any turbulence results in droplets travelling along particular streamlines not to be redistributed normal or transverse to the plane of flow through the homogenisation gaps. As mentioned above, there may be some degree of divergence when the fluid flows from a homogenisation gap to a pressure recovery zone, but the fluid divergence still only result in the divergence of streamlines in a plane normal to the flow through the homogenisation gap, and not in redistribution of droplets across streamlines. The droplets in the central zone of the fluid therefore remains substantially unchanged, whereas the droplet-size at the edges of the fluid is reduced to smaller diameters. Homogenisation is therefore not complete, and not efficient